The present invention is concerned with an alkali metal switch device.
Alkali metal electro chemical cells are known and a particular type which is the subject of much research and development work at the present time is the sodium/sulphur cell. In such cells, sodium metal provides the anode and sulphur/sodium polysulphides provide the cathode. The cell operates at a temperature at which the anode and cathode materials are liquid and the anode and cathode are separated by a solid electrolyte member made of an electronically insulating material which is conductive to sodium cations. A typical electrolyte material is beta alumina. In a fully charged cell, the cathodic material is substantially all sulphur. As the cell discharges, sodium cations migrate through the electrolyte to react with the sodium forming sodium polysulphides. The cell is fully discharged when substantially all sodium has migrated from the anode region or substantially all the sulphur has reacted to a lowest state sodium polysulphide. During recharging of the cell, sodium dissociates from the polysulphides and migrates back through the electrolyte to the anode region.
In order to provide the desired total storage capacity and a desired output voltage, sodium sulphur cells are interconnected in battery networks. If strings of series connected cells are required in the battery to provide the necessary output voltage, it is normal practice to interconnect corresponding cells of the different strings also in parallel, ideally with direct parallel connections between corresponding cells at the same potential in every series string. The need for this series/parallel connection of cells in a battery arises because of the possibility of individual cells becoming open circuit when either fully charged or discharged (or due to certain kinds of damage). If long series connected strings of cells are interconnected in parallel only at the ends of the string, then a single cell of the string becoming open circuit disconnects the entire string from the rest of the battery.
However, the multiple parallel connection of cells from different strings can make the battery more vulnerable if one cell sustains a short circuit failure, whereupon all parallel connected cells in the bank may discharge through the failed cell. As a result, the practice hitherto has been to use strings with several cells in series and interconnect these strings in parallel. The complete battery might then comprise several series connected sets of these parallel connected strings.
U.S. Pat. No. 4,414,297 discloses a shunt element which may be connected in parallel with a single cell in a series connected string of cells in a battery. The shunt element is normally open circuit when there is the usual operating voltage appearing across the cell with which it is connected in parallel. However, if the cell fails, losing its inherent output voltage and becoming a relatively high resistance, the continued flow of discharging current through the failed cell results in a reversal of the potential difference across the failed cell. In response to this opposite potential difference, the shunt device eventually becomes short circuit thereby providing a bypass circuit around the failed cell.
The shunt device comprises two regions containing liquid alkali metal and separated by a cationically conductive electronically insulating membrane. A first region has two current collectors one of which is constantly in contact with alkali metal remaining in the region, and the other of which is only contacted when the alkali metal reaches a predetermined level within the region. The second region has a current collector continuously in contact with alkali metal in the second region. The current collector in the second region is connected directly to the current collector in the first region which is not normally in contact with alkali metal in the first region. The current collector which remains constantly in contact with alkali metal in the first region is connected directly to a positive terminal of a cell to be shunted and the other two current collectors are connected to the negative terminal. Thus, whilst the cell is operative a positive potential is produced between the alkali metal in the first and second regions, tending to drive alkali metal into the second region. In the event of a reversal of polarity due to failure of the cell, alkali metal will be driven from the second region into the first region until the level of metal in the first region brings the metal into contact with the second current collector in the first region. This then produces a short circuit path through the alkali metal in the first region shunting the faulty cell.
It should be noted with the aforementioned prior art shunt element that, once the alkali metal level in the first region has risen to come into contact with the second current collector in the first region, the element then becomes substantially inactive apart from carrying any bypass current shunted round the failed cell. Because the current collector in the second region and the second current collector in the first region are connected directly together, no potential difference can be produced between the alkali metal in the two regions once the alkali metal reaches the level of the second current collector in the first region. As a result, the described element is only useful as a shunting element which can change irreversibly from an open circuit state to a closed circuit state.
Reference may be made also to GB-A-No. 1516638 which illustrates a sodium sulphur cell which incorporates an additional current collector located in the anode region and connected by means of a resistance to the current collector of the cathode region. This additional current collector in the anode region is contacted by alkali metal in the anode region only when the alkali metal reaches a level corresponding to a maximum level of charge of the cell, whereupon a bypass path is provided for further charging current via the resistance. This arrangement is provided to ensure that all cells of a series connected string of cells can reach maximum charge.